1. Field of the Invention
This invention relates to a magnesium based alloy. In particular, the invention relates to a magnesium alloy having superior mechanical properties at elevated temperatures. The alloy of this invention has excellent castability, and is particularly useful in die casting applications.
2. Description of Prior Art
The low density of magnesium, approximately 2/3 that of aluminum and 1/4 that of steel, makes it particularly attractive for transportation applications where weight reduction is critical. Magnesium is also surprisingly strong for a light metal; in fact, it has the best strength-to-weight ratio of any commonly available cast metal. In addition, magnesium can offer many other advantages such as good damping capacity, superior castability, excellent machinability, and good corrosion resistance. The use of magnesium alloy parts in automobiles has experienced a rapid growth in recent years due to the ever-increasing demand of vehicle weight reduction.
Magnesium alloy parts can be fabricated by the conventional casting processes including die casting, sand casting, plaster casting, permanent mold casting and investment casting.
Various alloys have been developed for use in particular applications including, for example, the die casting of parts for automobiles. Among these alloys, magnesium-aluminum based alloys, for instance AM50A and AM60B alloys ("AM" designates aluminum and manganese additions) containing about 5 to 6 wt. % of aluminum and a trace amount of manganese; and magnesium-aluminum-zinc based alloys, for instance AZ91D ("AZ" designates aluminum and zinc additions) containing about 9 wt. % of aluminum and about 1 wt. % of zinc, are economically priced and widely used in the fabrication of automobile parts. One disadvantage of these alloys is that they have low strength and poor creep resistance at elevated operating temperatures. This makes the above magnesium alloys unattractive for applications in the automotive powertrains where the components such as transmission cases will experience temperatures up to 150.degree. C. in the operating life. The poor creep strength of such components can lead to the reduction of fastener clamp load in bolted joints and, subsequently, to oil leakage in powertrains.
Another magnesium alloy which does provide some improved creep resistance is designated AE42 ("AE" designates aluminum and rare earth metal additions). This alloy comprises about 4 wt. % of aluminum and about 2 wt. % of rare earth elements. However, due to the use of rare earth elements, this alloy is difficult to die cast and uneconomical for volume production of automobile components.
Other magnesium alloys with good elevated-temperature properties have been developed over the years. These alloys can be classified into two groups. The first group of alloys contain exotic and expensive elements such as silver, yttrium, rare earth, and zirconium, and they are primarily developed for gravity sand casting and use in aerospace and nuclear reactors. The second group consists of a number of experimental alloys as disclosed in U.S. Pat. Nos. 4,997,662; 5,078,962; and 5,147,603. These alloys were developed for rapid solidification processes such as melt-spinning or spray deposition in which the extremely high solidification rates (10.sup.4 to 10.sup.7 K/sec.) can be achieved. Due to the high solidification rates, additions of certain alloying elements such as calcium or strontium can be made very high--up to 7 wt. %--contributing to the extremely high strength of these alloys at elevated temperatures. Unfortunately, the creep resistance of the alloys is poor because of the extremely fine grain structure in rapid solidification processed alloys. Another drawback of this group of alloys is that the process is not feasible for fabricating large components and is too costly for commercial production. None of alloys from the aforementioned groups is suitable for commercial die casting of automobile components.
The potential of adding calcium to magnesium-aluminum based die casting alloys for improved creep resistance has been investigated. British Patent No. 847,992 discloses that calcium additions from 0.5 to 3 wt. % can bring about high creep resistance to magnesium based alloys comprising up to 10 wt. % of aluminum, up to 0.5 wt. % of manganese and a possible zinc content of up to 4 wt. %. PCT/CA96/00091 discloses that magnesium based alloys containing 2 to 6 wt. % of aluminum and 0.1 to 0.8 wt. % of calcium show superior creep resistance at 150.degree. C. However, both documents acknowledge that alloys with high calcium contents are prone to hot-cracking during die casting. The British patent states that such hot-cracking tendency can be suppressed with considerable certainty or at least reduced to a fully satisfactory extent by ensuring that the iron content of the alloys is not less than 0.01 wt. % and preferably between 0.015 and 0.03 wt. %. However, it is now well known that such a high iron content will cause severe corrosion problems, as the tolerance limit for iron content in modern high-purity and corrosion-resistant magnesium alloys is 0.004 wt. %, as required by ASTM (American Society for Testing and Materials) Specification B93/B93M-94b. The PCT publication confirms that the use of calcium more than 0.8 wt. % adversely affects the die castability of the alloy due to extensive hot-cracking and die-sticking (also known as "die-soldering").
A third publication, entitled "Magnesium in the Volkswagen" by F. Hollrigl-Rosta, E. Just, J. Kohler and H. J. Melzer (Light Metal Age, 22-29, August 1980), discloses that outstanding improvement of creep resistance was provided by addition of about 1 wt. % calcium to a magnesium alloy AZ81 which contains about 8 wt. % of aluminum and about 1 wt. % of zinc. However, this publication discloses that the application of this alloy to the die casting production of crankcases (automotive parts) was not possible, because the castings stuck in the die and hot cracks occurred.
It is clear from the above three documents that the potential of improved creep resistance in magnesium alloys by calcium has not been fully realized due to the degraded castability associated with the calcium additions. Accordingly, there is a need in the art for economical magnesium alloys which exhibit improved castability while providing adequate creep strength.